Purifying cyclic esters by aqueous solvent extraction and further purification

ABSTRACT

A process is provided for the production and purification of cyclic esters in which the purification includes the introduction of an aqueous solvent into a cyclic ester containing composition and allowing two phases to form. A first phase includes cyclic esters and any organic solvent, and a second phase includes the aqueous solvent and impurities.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 08/473,400 filed Jun. 6, 1995, now U.S. Pat. No.5,686,630, which is a continuation-in-part of U.S. patent applicationSer. No. 08/417,597 filed Apr. 5, 1995, now U.S. Pat. No. 5,675,021,which is a continuation-in-part of U.S. patent application Ser. No.08/128,797, filed Sep. 29, 1993, now U.S. Pat. No. 5,420,304, which is acontinuation-in-part of U.S. patent application Ser. No. 07/854,559,filed Mar. 19, 1992, now U.S. Pat. No. 5,319,107, the disclosures ofwhich are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to processes for the production, recoveryand purification of cyclic esters and to cyclic ester compositions. In aparticular embodiment, the invention relates to the production, recoveryand purification of lactide and lactide compositions.

BACKGROUND OF THE INVENTION

Cyclic esters are useful in a variety of applications. For example,cyclic esters derived from hydroxycarboxylic acids are useful in thepreparation of environmentally biodegradable plastic materials, and forplastic materials which resorb when used in medical applications.Particularly useful are those polymers which are derived fromα-hydroxycarboxylic acids, such as lactic acid, because they can bedegraded over time by hydrolysis under most environmental conditions.The resulting hydroxy acid units (e.g., lactic acid) or oligomersthereof are then readily taken up by microorganisms in the environmentand converted to carbon dioxide and water aerobically or carbon dioxideand methane anaerobically.

Cyclic esters, such as cyclic esters of hydroxycarboxylic acids can beproduced through a number of mechanisms; however, such cyclic esterproducts typically contain multiple undesirable impurities. Suchimpurities can degrade cyclic esters in the product, resulting in shortshelf life of the cyclic esters produced. For example, free acid andwater in a cyclic ester composition can propagate hydrolyzing of thecyclic ester bonds, degrading the cyclic ester back intohydroxycarboxylic acids or other degradation products. In addition,impurities in the cyclic ester composition can interfere with both therate of polymerization of the cyclic ester and the molecular weight,thereby prohibiting the formation of desirable higher molecular weightpolymerization products. Accordingly, there is a need to obtainsubstantially pure cyclic esters, which are substantially free fromimpurities which degrade the compounds or interfere with subsequentchemical reactions, such as polymerization of the cyclic esters intohigh molecular weight polymers.

Several processes are known to purify synthesized cyclic estercompositions. Such processes include solvent crystallization, solventscrubbing, solvent extraction, distillation, melt crystallization andsublimation. Although satisfactory results can be obtained in controlledlaboratory conditions, many processes are difficult to operate on acommercial scale because they are complex or have impractical operatingparameters. In addition, such processes may require high energy costs,high equipment costs or high reagent costs. Moreover, machinery may notbe available for implementing these processes on a commercial scale.

Other processes are commercially unacceptable because the cyclic estersbeing purified may degrade due to the residence time or temperatureconstraints of the process, resulting in poor purity. Further, someprocesses provide low yields, making them economically unsuitable.

Accordingly, there is a need for inexpensive, reliable methods forpurifying cyclic esters. In particular, there is a need for methods ofobtaining cyclic esters of sufficient purity for use in producing highmolecular weight polymers. Furthermore, there is a need for methods ofpurifying cyclic esters that allow for recovering all isomeric forms ofthe cyclic esters.

SUMMARY OF THE INVENTION

The present invention is directed to a process for producing a cyclicester by the steps of (a) providing a feedstream comprising an organicsolvent and compounds selected from the group of a singlehydroxycarboxylic acid or its ester, salt or amide and oligomericspecies thereof; (b) removing water from the feedstream to form aproduct stream which includes the cyclic ester, wherein theconcentration of pentamers and higher oligomers in the product stream isless than about 20% by weight of the reaction mixture during theprocess; and (c) introducing an aqueous solvent into the product streamand allowing the product stream to separate into a first phasecomprising cyclic esters and a second phase comprising the aqueoussolvent and impurities.

In another embodiment, the present invention is directed to a processfor the purification of cyclic esters in a composition comprising cyclicesters, an organic solvent and impurities. The process includesintroducing an aqueous solvent into the composition and thereafterallowing the composition to separate into a first phase which includesthe cyclic esters and organic solvent and a second phase which includesthe aqueous solvent and impurities. In one embodiment, the amount ofaqueous solvent introduced into the composition is less than about 3%based on the weight of the composition above the mutual solubility limitof water in the organic solvent. In another embodiment, the amount ofaqueous solvent introduced into the composition is less than an amountnecessary to effect hydrolysis of oligomeric XA species in thecomposition.

A further embodiment of the invention includes a process forpurification of cyclic esters in a composition which includes cyclicesters, an organic solvent and impurities. This process includesintroducing an aqueous solvent into the composition and allowing thecomposition to separate into a first phase comprising the cyclic estersand organic solvent and a second phase comprising the aqueous solventand impurities. The process further includes contacting the first phasewith a first adsorbent to remove impurities.

The various embodiments of the present invention allow for the effectiveproduction, recovery and purification of cyclic esters by providingprocesses which can be efficiently integrated. For example, the use of awater-assisted separation purification step allows for the upstream useof a wider selection of cyclic ester production solvents, as well asreduces the load of impurities to be removed in subsequent purificationsteps.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a flow diagram of an embodiment of an integrated cyclic esterproduction and purification process; and

FIG. 2 is a flow diagram of another embodiment of an integrated cyclicester production and purification process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention has utility in the production and purification ofcyclic esters. Such compounds are useful for the production of polymers.For example, a cyclic ester such as lactide is useful in the productionof polylactic acid. In particular, the present invention relates toproduction and purification of cyclic esters substantially free ofimpurities that can degrade the cyclic esters or interfere with thepolymerization of the cyclic esters into high molecular weight polymers.

The process of the present invention involves the production andpurification of cyclic esters derived from hydroxycarboxylic acids,hydroxycarboxylic acid esters, hydroxycarboxylic acid salts, orhydroxycarboxylic acid amides. The term "derived from" refers to thecyclic ester being produced by reactions in which these components orproducts of these components are reactants. Cyclic esters can be formedby converting an ester formed from any two hydroxycarboxylic acids,esters, salts, or amides thereof, into a cyclic diester. Cyclic esterscan also be intramolecular esters or a cyclic monoester, such aslactones. Cyclic esters are referred to herein as XD. X₁ A refers to ahydroxycarboxylic acid, hydroxycarboxylic acid ester, hydroxycarboxylicacid salt, or hydroxycarboxylic acid amide. X₂ A refers to a lineardimer molecule of an X₁ A molecule. X₃ A refers to a linear trimermolecule of an X₁ A molecule, and X_(n) A refers to a linear n-mermolecule of an X₁ A molecule. XA without subscript denotes one or moreof X₁ A, X₂ A, X₃ A, and X₄ A or a solution containing those species. Itwill be understood that when X is substituted by L, the correspondingcompounds based on lactic acid are meant. For example, LA refers to alactic acid-based mixture, including L₁ A, L₂ A, L₃ A and L₄ A, and LDrefers to the cyclic dimer of lactic acid, e.g., lactide.

Cyclic esters of the present invention include those compounds describedabove. Preferred X₁ A species for production of cyclic esters include,but are not limited to, the following acids and corresponding esters,salts, or amides thereof: lactic acid, glycolic acid, tartaric acid,mandelic acid, malic acid, 1-hydroxy 1-cyclohexane carboxylic acid,2-hydroxy-2-(2-tetrahydrofuranyl) ethanoic acid, 2-hydroxy-2-(2-furanyl)ethanoic acid, 2-hydroxy-2-phenylpropionic acid,2-hydroxy-2-methylpropionic acid, 2-hydroxy-2-methylbutanoic acid,2-hydroxybutanoic acid, 2-hydroxypentanoic acid, 2-hydroxycaproic acid,2-hydroxyoctanoic acid, and mixtures thereof.

Specific cyclic ester production processes of the present invention aredescribed in more detail below. Such production processes, however, canbe production of cyclic esters by direct formation of a cyclic esterfrom an X₂ A species, such as is described in U.S. Pat. No. 5,319,107.Alternatively, production of cyclic esters can be by other knownmethods, such as depolymerization of linear esters of lower molecularweight or synthesis by reactions of α-halo salts of carboxylic acids.

PURIFICATION OF CYCLIC ESTERS Adsorption Treatment

The present invention includes the removal of impurities from a feedmaterial containing cyclic esters. Such impurities include, for example:water, XA (e.g. free acids), metal ions, mineral acids, cyclic estersynthesis catalysts and associated catalyst degradation products, andtypical fermentation acids and alcohols such as formic acid, aceticacid, propionic acid, butyric acid, ethanol and butanol, all of whichcan contribute to the degradation of cyclic esters or interfere withsubsequent polymerization of cyclic esters into useful high molecularweight polymers.

The feed material can also include a solvent in which cyclic esters havegood solubility, but which does not degrade the cyclic esters such as bybeing reactive with cyclic esters. Such solvents include those solventsused in cyclic ester production and recovery as described below.Examples of such solvents include, but are not limited to, xylene,toluene, benzene, anisole, methyl isobutyl ketone (MIBK), isopropylether and mixtures thereof, and a preferred solvent comprises xylene.Solvents which are not preferred because of being reactive with cyclicesters include alcohols, organic acids, esters and ethers containingalcohol, peroxide and/or acid impurities, ketones and aldehydes with astable enol form and amines.

The relative concentrations in the feed material of the cyclic esters,impurities, and solvent depend upon the process used for producing thecyclic esters. Typically for direct synthesis of cyclic esters from X₂A, the feed material will contain from about 0.5 to about 10% by weightcyclic esters, less than about 10% by weight impurities, and from about80 to about 99% by weight solvent. The impurities generally include lessthan about 1% by weight water and less than about 10% by weighthydroxycarboxylic acid and its ester, salt and amide.

In the purification method of the present invention, the feed materialis subjected to an adsorption step in which at least one impurity in thefeed material is adsorbed from the feed material by an adsorbent,thereby concentrating and purifying the cyclic ester component of thefeed material. The term "adsorption" refers to the attraction orinteraction between an impurity in a feedstream and an adsorbent.Preferably, this attraction or interaction between an impurity and anadsorbent is a non-covalent attractive force which is reversible. Suchattraction, for example, can be based on charge differences ordifferences in charge distribution between the adsorbent and theimpurity such as is the case in the capture of a molecular species by anion exchange resin. Included in this definition is the concept of ionexchange in which charged species are reversibly exchanged on thesurface or within the structure of an adsorbent. As discussed in moredetail below, in the process of the present invention the resultingpurified solution which contains purified cyclic esters is then treatedby a post-adsorption treatment, such as to recover the cyclic esters ina solid form or to use the cyclic esters directly such as bypolymerization.

Adsorbents suitable for use in the present invention include ionexchange resins, molecular sieves, alumina, silica gel, activatedcarbon, clays and other adsorbents known in the art. Preferredadsorbents are ion exchange resins, molecular sieves, silica gel,activated carbon and clays, and particularly preferred adsorbentsinclude ion exchange resins and molecular sieves.

Selection of a specific adsorbent for a particular process depends onthe type of impurity which is to be removed from the feed material. Forexample, if the impurity is an XA acid, it is generally desirable toremove the impurity from the feed material by contacting it with eithera weak or a strong anion exchange resin. Preferably however, to removeacid impurities such as XA from feed material, it is desirable to useweak anion exchange resins. Weak anion exchange resins can achieveacceptable rates of removal of impurities while also allowing relativelyeasy regeneration of the resin. For example, suitable commerciallyavailable weak anion exchange resins are sold by Reilly Industries,Inc., Indianapolis, Ind., under the product designation REILLEX® 425 andby Rohm and Haas, Philadelphia, Pa., under the product designationAMBERLYST® A-21. REILLEX® 425 is a macroreticular bead form ofcrosslinked poly-4-vinylpyridine. AMBERLYST® A-21 is adivinylbenzene-styrene based resin with tertiary amine functionalgroups.

To remove water as an impurity from the feed material, the feed materialcan be contacted with an adsorbent selected from the group consisting ofmolecular sieves, alumina and silica gels, and most preferably molecularsieves. As used herein, the term "molecular sieve" refers to anadsorbent having structural characteristics which promote adsorption ofimpurities and which excludes the adsorption of cyclic esters.

For the removal of XA amides as impurities, it is generally desirable touse complexing resins. For the removal of XA salts as impurities, it isgenerally desirable to use a strong anion exchange resins.

For removal of alcohols as impurities, it is generally desirable to usemolecular sieves or ion exchange resins. Since these adsorbents havestronger affinities for the more polar impurities in the feed such as XAand H₂ O, it is generally desirable to remove these more polarimpurities from the feed prior to alcohol adsorption treatment. Removalof these more polar impurities allows the lower affinity alcohols toadsorb on these materials without having to compete with more polar highaffinity impurities in the feed, thus increasing the capacity of thesealcohol adsorbents.

The step of contacting a feed stream with an adsorbent can be conductedat any suitable temperature such that acceptable rates of adsorptionoccur. Preferably, the temperature of the feed stream is between about0° C. and about 100° C., and more preferably between about 25° C. andabout 45° C.

In a preferred embodiment of the adsorption process of the presentinvention, feed material is subjected to a first adsorption to remove afirst impurity and then is subjected to a second adsorption to remove asecond impurity. Optionally, additional adsorption steps can beconducted. The purified cyclic ester-containing feed material can thenbe subjected to post-adsorption treatment to recover cyclic esters asdescribed below. The use of such multi-step adsorption processes isparticularly useful in the present invention because they provide forthe removal of multiple impurities from the feed material. For example,a first adsorption can be conducted to remove impurities such as XA fromthe feed material and a second adsorption can be conducted to removewater from the feed material, or vice versa. A third adsorption canoptionally be conducted, for example, to remove alcohols if present.Further, a fourth adsorption can optionally be conducted, for example,with a carbon filter to remove color bodies if present. In the case ofmulti-step adsorption, the steps can be conducted in any desirableorder.

In a preferred embodiment, adsorbents suitable for use in the presentinvention are heat stable at temperatures above about 100° C., and morepreferably, above about 130° C. The term "heat stable" refers to theability of an adsorbent to retain its capacity, when subjected to hightemperatures, to adsorb impurities upon return to normal adsorptionoperating temperatures. For example, while REILLEX® 425 resin may beused for adsorbing impurities at temperatures of less than about 100°C., it is thermally stable at temperatures up to about 260° C. Thus,such resins are particularly useful in a purification process in whichthe resin is regenerated at elevated temperatures after adsorption atnormal operating temperatures.

Preferred adsorbents of the present invention include adsorbents whichdo not degrade cyclic esters in the feed material. For example,processes of the present invention are conducted with adsorbents andunder conditions such that preferably less than bout 25% by weight ofcyclic esters are degraded by contact with the adsorbent, morepreferably less than about 10% by weight and most preferably less thanabout 5% by weight. In this regard, an adsorbent such as an ion exchangeresin is preferred over an adsorbent such as alumina which can degradecyclic esters.

Adsorption steps of the present invention can be accomplished using avariety of apparatus such as a fixed adsorbent using packed columns orbeds as are known in the art. Some examples of these types of systemsare: moving bed, simulated moving bed, two column arrangements and threecolumn arrangements. In addition, it is possible to use an adsorbentwhich is loose in the feed material such as being a slurry formed fromthe adsorbent material and the feed material.

Adsorption operations of purification processes of the present inventioncan be conducted in either batch or continuous modes. It is preferred,however, that the such adsorption methods be operated continuously.

In a further embodiment of the present invention, the adsorptiontreatment can include adsorption treatment of an evaporated solventstream produced by evaporation prior to a main crystallizationpurification process. For example, it is known that cyclic esters can bepurified by a variety of crystallization processes, including solventcrystallization. Prior to a conventional solvent crystallizationprocess, a cyclic ester-containing product stream which additionallyincludes impurities and solvent can be concentrated by evaporating aportion of the solvent. In such an instance, the evaporated solventproduced thereby includes small amounts of cyclic ester and impurities.Thus, the evaporated solvent stream can be purified by adsorptiontreatment as broadly described herein prior to reuse of the evaporatedsolvent stream in other unit operations. For example, the evaporatedsolvent stream can be subjected to one or more adsorption steps, such asan acid adsorption step and a water adsorption step, to remove acid andwater impurities. The resulting solvent stream would include solvent andremaining cyclic esters which are not removed by the adsorption steps.Such a stream can then be reused in other operations in an overallrecovery process. For example, such a solvent stream can be used assolvent for a solvent crystallization process of the concentrated cyclicester composition resulting from the initial evaporation of solvent.Such a crystallization can be a multiple stage recrystallizationprocess.

The solvent resulting from the solvent crystallizations can be furtherrecycled to other points in the overall recovery and purificationprocess for cyclic esters. For example, such a solvent stream, whichwould include impurities and cyclic ester, can be recycled to aliquid-liquid phase separation step prior to concentration of a cyclicester-containing product stream.

Post-Adsorption Treatments

As noted above, subsequent to purification by adsorption, a variety ofpost-adsorption treatments can be employed to either recover the cyclicesters in some solid form or to directly polymerize cyclic esters.Recovery processes include concentration processes, drying processes andfurther purification, among others known in the art.

In one embodiment of recovery, cyclic esters can be recovered by spraydrying the purified feed material in order to recover the cyclic estersin powder form. Spray drying involves the atomization of the purifiedfeed material in countercurrent flow with a drying, preferably dry andinert, gas to drive off solvent from the purified feed material.Typically, spray drying is conducted at elevated temperatures. Spraydrying results in the production of a powder which can be packaged andsold and is suitable for use such as in the production of polymericmaterial.

In another post-adsorption treatment, the purified feed material can beconcentrated, such as through the use of an evaporator. For example, thepurified feed material can be first run through an evaporator unit inwhich solvent is driven off. In preferred embodiments, enough solvent isdriven off so that the remaining cyclic ester mixture has between about1% and about 80% by weight solvent, more preferably between about 5% andabout 50% solvent, and even more preferably between about 15% and about30% solvent. It shall be noted that solvent which is removed duringconcentration can be recycled, for example, back to other stages in anoverall process, including, for example, recovery of X₁ A prior tocyclic ester production, regeneration fluid for acid adsorption units,or directly back to cyclic ester production operations as described inmore detail below. Such solvent can also be brought forward for use insubsequent recovery or purification operations, such as solventcrystallizations. After exiting the evaporator unit, the cyclicester-containing stream can be cooled prior to additional recovery orpurification operations.

After concentration, the concentrated, purified feed material can bepolymerized directly or dried as a final product. In the embodiment ofdrying concentrated cyclic ester material, the concentrated feedmaterial can optionally be further purified prior to drying. Drying canbe accomplished by numerous methods, such as by spray drying theconcentrated, purified feed material as discussed above. Further,concentrated cyclic esters can be prilled. Prilling refers to a processfor pelletizing a solid material which includes melting the material andspraying the molten material, whereby droplets of the material solidify.Prilling involves the atomization of an essentially solvent free, moltenpurified feed material in countercurrent flow with a cooling, preferablydry and inert, gas to cool and solidify the purified feed material.Typically, prilling is conducted at near ambient temperatures. Prillingresults in the production of beads which can be packaged and sold and issuitable for use such as in the production of polymeric material.

As noted, further purification of the recovered cyclic esters can beconducted before drying, such as by solvent cooling crystallization,solvent evaporative crystallization, melt crystallization, distillation,or combinations thereof. All of these methods are classified as positivepurification steps since the desired cyclic ester species preferentiallyundergoes a phase change and is removed from the impure feedstock. Incontrast, the adsorption based purification discussed above areclassified as subtractive purification steps since they remove specificimpurities from the impure feedstock. While the subtractive purificationsteps can drive the levels of impurities, such as acids, water, andalcohols to very low levels, it is possible that other undesirablespecies could pass through the adsorption steps and end up in the cyclicester product if a positive purification step is not used. For example,waxy residues commonly found in industrial grades of solvents, such asxylene, would not be removed by adsorption treatments. If thepost-adsorption treatments only consisted of evaporation and spraydrying or prilling, these waxy residues would end up in the cyclic esterproduct used for polymerization. While this particular contaminate maynot affect the polymerization rate and molecular weight results, it doesnegatively affect assay values and appearance of the cyclic esterproduct both of which are commercially important. As another example, XAobtained from fermentation based processes will typically contain lowlevels of poorly defined contaminants which could also pass through theadsorption purification steps. For these reasons, it is often desirableto include one of these positive purification steps in thepost-adsorption treatment processing.

There are several advantages for using the adsorption steps prior to thepost-adsorption purification treatment steps. For example, in theinstance of post-adsorption crystallization, larger crystals can beproduced because crystal size usually increases when crystallizationsare performed on solutions with low levels of impurities. Large crystalsare easier to handle, have lower surface area to volume ratios, and aregenerally more pleasing to potential customers of cyclic ester products.As another example, by removing the XA and water by adsorption prior topositive purification steps, one can use higher temperature processingin the post-adsorption treatments without significant degradation of thecyclic ester product. Thus, for example, high temperature processingsteps like prilling and distillation can be used with less degradationthan if significant XA or water were present. The following discussionof positive purification type post-adsorption treatments steps isordered by increasing levels of high temperature processing.

In one embodiment the concentrated feed material can be solventcrystallized to obtain a purified cake and then further dried to removeresidual solvent. There are numerous configurations of equipment whichcan be used for both solvent cooling and solvent evaporativecrystallization. Those familiar with the art will be able to pick anappropriate configuration for a given application.

Solvent used in solvent crystallization performs a number of functionsduring the crystallization process. By use of a solvent, crystallizationcan be conducted at lower temperatures than if no solvent is present. Inaddition, the presence of solvent reduces viscosity of the systemthereby making material handling and pumping easier and improving heattransfer. Further, the presence of the solvent can result in a purercrystallization process by providing a medium to contain impurities suchas X₁ A and oligomers of X₁ A during crystallization. Thus, uponsubsequent separation of crystals from the crystallization mixture,impurities such as X₁ A and oligomers of X₁ A can more readily separatewith the liquid stream, rather than adhering to crystals.

The liquid stream resulting from separating the cyclic ester crystalscan be further treated for additional recovery of cyclic esters from theliquid stream. Additionally, the liquid stream can be treated forrecycle of X₁ A and oligomers of X₁ A present in the liquid.

In an alternative embodiment, the cyclic esters can be further purifiedby melt crystallization. In a melt crystallization process, the cyclicester material is subjected to temperatures sufficient to melt thecyclic esters without the presence of significant amounts of solvent.The melted material is then cooled and a portion of the material isallowed to crystallize. There are several configurations of equipmentfor melt crystallization. Those familiar with the art will be able topick an appropriate configuration for a given application.

It should be noted that melt crystallization has a number of particularadvantages over solvent crystallization processes. For example, becausethe volume of material being handled is significantly smaller in theabsence of a solvent, smaller sized equipment is needed to obtain thesame production. In addition, it has been found that larger crystalsizes are obtained by melt crystallization. Larger crystals aretypically more pure than smaller crystals due to higher volume tosurface ratios, thereby reducing surface area available for adherence ofimpurities.

A further embodiment of the present invention when post-adsorptiontreatment includes an additional purification step such ascrystallization is the selective recovery of cyclic esters from amixture which includes more than one isomeric species of the cyclicester. This process includes selectively crystallizing one of theisomeric species of the cyclic ester and recovering that isomericspecies. This recovery method is suitable when X₁ A is a chiral moleculeand, thus, has isomeric forms. For example, lactic acid is a chiralspecies of X₁ A. There exist two optical isomers of lactic acid,L-lactic acid and D-lactic acid. Consequently, lactide can be eitherL-LD (a lactide molecule formed from two L-lactic acid molecules) , D-LD(a lactide molecule formed from two D-lactic acid molecules), meso-LD(e.g., M-lactide, a lactide molecule formed from one L-lactic acidmolecule and one D-lactic acid molecule), or D,L-LD (an intermolecularspecies consisting of one L-LD molecule and one D-LD molecule). Thedifferent species of lactide have different melting points. Meso-LD hasthe lowest melting point of 52.8° C., isomerically pure D-LD and L-LDboth have melting points of 98.7° C., and pure D,L-LD has the highestmelting point of 128° C.

For example, isomeric species of a given XD molecule having highermelting points than other isomeric species can be selectivelycrystallized in a melt crystallization process. By forming a melt of theentire cyclic ester mixture, and crystallizing the higher melting pointspecies at a temperature above the melting point of the lower meltingpoint cyclic ester species, the higher melting point species can beselectively crystallized. Alternatively, an isomeric species of a givenXD molecule having higher melting points than other isomeric speciesusually will be less soluble in any given solvent. By conducting asolvent crystallization at a temperature above the solubility limit ofthe lower melting point cyclic ester species, the higher melting pointspecies can be selectively crystallized.

Subsequently, upon recovery of the crystals, for example, bycentrifugation, the resulting solid will have the higher melting pointisomeric species plus residual amounts of the lower melting pointisomeric species. The cake from this crystallization can be potentiallyused as one of the two XD feeds to an XD polymerization step. The otherfeed to the XD polymerization step could be recovered from the liquidresidue produced by the selective crystallization step using varioustechniques such as spray drying, prilling, crystallization,distillation, evaporation, or combinations thereof.

It should be noted that selective crystallization, as discussed above,will not achieve 100% selectivity. Thus, in the example discussed above,the first crystallized fraction may contain residual amounts of thelower melting point species. Likewise, the second recovered fractionwill contain significant amounts of the higher melting point species.However, as long as the desired ratio of optical isomers for thepolymerization operations is between the isomer contents of the two XDproducts, a simple blending operation can be used to adjust the actualratio of optical isomers in the XD feedstock for the polymerizationoperations. This is significant since it is one possible method forcontrol over the optical isomer content of the XD based polymers, whichis what controls the physical and degradation properties of many XDbased polymers.

In an alternative embodiment of post-adsorption processing, the cyclicesters can be further purified by distillation. Typically, a center cutwould be taken as product in a batch distillation process.Alternatively, a center cut could be generated in a continuousdistillation system using a standard two column method or a one columnmethod utilizing a sidestream draw for the product stream.

The product from the distillation process can be used directly in XDpolymerization. Alternatively, the product could be spray dried orprilled, and then stored for later use.

A further embodiment of the present invention when post-adsorptiontreatment includes an additional distillation purification step is theselective isomeric recovery of the cyclic esters. For example, theboiling points of L-LD, D-LD, and D,L-LD are all very close anddistillation would not be used to separate these species. However, theboiling point of M-LD is significantly lower than the boiling points ofthe other isomers, thus a distillation system could be designed toproduce two product fractions with significantly different isomercontents. This represents another method for controlling the isomercontent of the XD polymerization unit feedstock, thus controlling theisomer content at the XD based polymers.

It should be clear from the previous discussion that one of the majorissues in cyclic ester recovery processes is how to provide control ofthe isomer content of the cyclic ester products used for polymerization.One major advantage of the adsorption treatments is that they arenon-stereospecific purifications, thus the isomer content of the productof the adsorption treatment steps is the same as the isomer content ofthe feed to the adsorption treatment steps. All of the non-purificationbased post-adsorption treatment steps discussed here are alsonon-stereospecific processing steps. Thus for example, if one were touse concentration followed by spray drying for the post-adsorptiontreatment steps, the isomer content of the cyclic ester product would bedetermined by the isomer content of the feed to the adsorptionpurification steps. This outcome is significant since isomer control caneasily be performed by either controlling the isomer ratio of the XAfeed to the synthesis reactor, or by controlling the degree ofracemization occurring in the XD synthesis reactor by changingtemperature, residence times, levels of racemizing agents added to thereactor, etc., or by combinations of XA feed isomer control and XDsynthesis reactor conditions.

The purification based post-adsorption treatment steps discussed herehave varying degrees of stereospecificity. Crystallization based methodsgenerally produce purified products containing only one or twostereoisomers of the cyclic esters. As described previously, the otherisomers can be recovered from the liquid residues of the crystallizationsteps. Alternatively, one could use a blocked operation mode with acrystallization based method in which the entire XD synthesis andrecovery operation is timeshared between producing the various desiredisomers of the cyclic ester products. The distillation based methods aregenerally not as stereospecific as the crystallization methods. Forexample, the boiling point of M-LD is close enough to the other isomersthat one could design for either a one product or two product system.The isomer content of the one product system would essentially be thesame as the isomer content of the feed to the adsorption unit. The twoproduct distillation system described previously would allow one toindividually control the isomer content of multiple polymerization lineoperating in parallel.

Adsorbent Regeneration

The purification processes of the present invention can also includeregeneration of the spent adsorbent wherein the used adsorbent istreated in order to remove or "desorb" at least a portion of theadsorbed impurity and subsequent reuse of the regenerated adsorbent foradsorption. Numerous regeneration techniques are suitable, and dependupon the type of adsorbent being used. Generally, regeneration requirescontacting the adsorbent with a fluid (gas, liquid, or supercritical)under conditions which cause the adsorbed impurities to desorb from theadsorbent into the fluid.

In the instance where the impurity being desorbed from an adsorbent iswater being desorbed from, for example, a molecular sieve, thedesorption step is a drying process. Such drying processes can beaccomplished by drying the adsorbents using known techniques. Forexample, a typical drying process includes contacting the adsorbent witha hot, preferably inert gas to remove the entrapped water. Drying can beconducted at any suitable temperature and is preferably conducted at atemperature between about 175° C. and about 300° C. depending upon thetype of molecular sieve being used. Drying can optionally be carried outin a reduced pressure environment to accelerate the evolution of water.

In the instance where the impurity being desorbed from an adsorbentincludes an XA species and/or XA oligomers, the desorption fluid can bean organic solvent, an aqueous solution or a hot inert gas. For example,a solvent which was the solvent originally in the feed material can beused for desorption of impurities from the resin. Alternatively, othersolvents in which the impurities are soluble are acceptable fordesorption and can include, but are not limited to those identifiedabove in the discussion regarding adsorption treatment. A preferreddesorption solvent is xylene.

In an optional embodiment, the desorption fluid can be heated prior tocontact with the adsorbent to improve its effectiveness. Contacting theheated desorption fluid with the adsorbent causes an increase in thetemperature of the adsorbent. This in turn causes a decrease in theadsorption equilibrium so a portion of the absorbed XA species willdesorb from the adsorbent. The amount of XA desorbed by heating willdepend upon the amount of the adsorbent temperature rise and thetemperature sensitivity of adsorption equilibrium for a given adsorbentand solvent combination. Typically, the adsorption equilibrium for weakabsorbents such as weak anion exchange resins are more sensitive totemperature changes when compared to strong absorbents.

Other means can be used to heat the adsorbent. For example, if theadsorbent is installed in the tubes of a shell and tube heat exchanger,the adsorbent can be heated by applying steam or other heat transfermedia to the shell. In this configuration, a desorption fluid still hasto be used to carry the XA species away from the resin, however theamount of desorption fluid required is generally less than the adiabaticheating cycle described in the previous paragraph.

Temperatures for regenerating adsorbents are generally in the range ofabout 100° C. to about 260° C. Suitable temperatures can be selectedsuch that acceptable rates of desorption are achieved without causingsubstantial degradation or loss of adsorption capacity. In the event ofa desorption temperature being in excess of the boiling point of thedesorbing fluid, the desorption can be conducted under pressure.

The main advantage of the thermal regeneration methods is that there areno waste products formed except for XA species in a solvent of choice.Thus the regeneration effluent streams can easily be recycled to severalplaces in the cyclic ester synthesis process with little or noadditional treatment. Thermal regeneration methods, however, are fairlygentle and do not desorb strongly adsorbed species from the resin. Thesestrongly adsorbed species typically would include metal ions, mineralacids, and catalyst degradation products such as sulfonated xylene. Overtime, these strongly adsorbed species may decrease the XA capacity ofthe resin to the point where a more powerful desorption technique isrequired.

Chemical regeneration methods can also be used to desorb the XA speciesfrom the resin. These methods use more powerful desorption agents whencompared to the thermal methods. In chemical regeneration, a causticsolution is used to desorb the XA and other strongly adsorbed speciesfrom the resin. The caustic solutions can either be aqueous based oralternatively the caustic can be dissolved in polar organic solventssuch as methanol. While chemical methods are more powerful, their useraise some additional issues. First, at the end of the desorption cyclethe resin will contain a caustic solution which has to be removed fromthe resin prior to the next adsorption cycle. Since these solutionscontain caustic, water, and/or alcohols, all of which are detrimental ineither cyclic ester recovery or polymerization processes, fairlyrigorous removal of these species is required. Second the effluents fromthe caustic regeneration will generally contain salts which must betreated prior to recycling or disposal. This treating equipment can be aseparate set of equipment located within the XD production andpurification unit. Alternatively, if the XA feedstock to the synthesisreactor is derived by acidifying a fermentation broth, the regenerationeffluent can be sent to the acidification step in fermentation recoveryfor an integrated XA and XD production facility.

Combinations of the thermal and chemical regeneration methods could beused. For example, thermal regeneration could be used in manyregeneration cycles until the adsorption capacity of the resin fallsbelow a desired minimum value. Then, one or more chemical basedregeneration cycles would be used to restore resin capacity. Then,thermal regeneration cycles could be used again until the adsorptioncapacity falls below the desired performance.

In another aspect of the invention, various guard column configurationscan be used upstream of a thermally regenerable XA adsorption unit. Thepurpose of the guard column is to selectively adsorb the stronglyadsorbing species from the feed and allow the XA species to pass throughto the thermally regenerable XA adsorption column. This would greatlydecrease the frequency at which the XA adsorption column would have touse chemical regeneration methods. Another advantage of this is that,since the guard column would have to be chemically regenerated anyway,the thermal stability of the resins used in the guard column is not anissue. Thus, inexpensive, high capacity resins with low thermalstability could be used for the guard column. As an example of a goodcandidate for the guard column resin, the AMBERLYST® A-21 resindescribed previously is inexpensive, has six times the capacity of theREILLEX® 425, but only has an upper temperature stability limit of about75° C. On the other hand, REILLEX® 425 is more expensive, has a lowercapacity, but is thermally regenerable at temperatures up to 260° C.,thus making this resin a reasonable choice for use in the XA adsorptionapplication.

The desorbing fluid which contains desorbed impurities can be recycledfor use in other stages of an overall cyclic ester production andrecovery process. The destination of such recycle streams will depend onthe nature of the desorbing fluid and the impurities in it. For example,if the desorbing fluid is an organic solvent such as xylene and theimpurities are XA species, small amounts of cyclic ester will begenerated during the recycle process. Thus, the recycle stream can bedirected to a post-cyclic ester production stage prior to adsorption.For example, as described below in more detail a cyclic ester productionprocess (an include a phase split or extraction step after cyclic esterproduction and before adsorption. By recycling a regeneration stream tosuch a phase split/extraction step, (1) generated cyclic ester can berecovered in the adsorption purification, and (2) oligomers and XA acidspecies will be separated and for example, can be sent to a hydrolysisunit prior to the cyclic ester production process.

In the event the desorbing fluid is water, for example, the desorptionrecycle stream can be recycled directly to a hydrolysis unit prior tothe cyclic ester production process. In this manner, hydrolysis ofoligomers in the recycle stream can be initiated by the water therebyreducing the required capacity of the hydrolysis unit.

In continuous processes which involve adsorbent regeneration, two ormore adsorbent columns or beds can be used in order to alternate theiruse to allow the continuous operation of the process. When oneadsorption column or bed has reached capacity, the column or bed istaken off-line and a new regenerated column or bed is switched in foradsorption duty.

PURIFIED CYCLIC ESTER COMPOSITIONS

A further aspect of the present invention includes highly pure cyclicester compositions. Such compositions can be produced by the methods ofthe present invention. Cyclic ester compositions such as lactide areunstable in the presence of water and acid. In the presence of water,lactide can be hydrolyzed to L₂ A which can be hydrolyzed to L₁ A.Further, free acid can catalyze the hydrolysis reaction. Thus, cyclicester compositions in accordance with the present invention having lowlevels of water and free acid are highly advantageous because of theresulting stability during shipping and storage and extended shelf life.

For every 10 ppm of water in a lactide product which is consumed byhydrolysis reactions, a free acid increase of 0.56 meq free acid/kg LDis incurred. Typical specifications for commercial lactide are a maximumof 200 ppm of water and a maximum of 1 meq free acid/kg LD. Thus, evenfor lactide products which initially meet product specifications, morethan enough water can be present to generate free acid levels aboveacceptable levels during subsequent storage and processing. Therefore,it should be clear that current commercial products do not have a longshelf life because with such high levels of water, the hydrolysisreactions will cause the free acid levels to rise above specificationlevels in a relatively short time.

In accordance with the present invention, cyclic ester compositions areprovided which have water contents of less than about 200 parts permillion (ppm), more preferably less than about 60 ppm and mostpreferably less than about 20 ppm. Further, in accordance with thepresent invention, cyclic ester compositions are provided which havefree acid contents of less than about 1 milliequivalent per kilogram ofcyclic ester (meq/kg), more preferably less than about 0.10 meq/kg andmost preferably less than about 0.04 meq/kg. As noted above, water andfree acid in cyclic ester compositions interact to degrade cyclicesters. Thus, the present invention includes cyclic ester compositionshaving both low water and low acid levels as are identified above.

A further aspect of the cyclic ester compositions of the presentinvention includes a packaged cyclic ester product which is highlyimpermeable to water because of the extreme water sensitivity ofcontained cyclic ester products. Such products have low water and acidcontents and are packaged in packages which prevent significant amountsof water from passing into the interior of the package containing thecyclic ester composition. In particular, cyclic ester compositions ofthe present invention are packaged in packages having water vaportransmission rates less than about 0.1 g/(100 in²) (24 hr), morepreferably less than about 0.01 g/(100 in²) (24 hr), and most preferablyless than about 0.001 g/(100 in²) (24 hr) at 100° F. and 90% relativehumidity.

Suitable packages and packaging techniques to meet the requirementsidentified above are known to those skilled in the art. For example,high quality foil-LDPE laminates can have water vapor transmission ratesas low as 0.006 g/(100 in²) (24 hr) at 100° F. and 90% relativehumidity. Metallized films are also known to have low water vaportransmission rates. Packaging systems using double bagging with adesiccant placed between the bags can be used or even higher water vaporbarrier packaging systems such as metal containers or glass containerscan also be used. In addition, storage techniques such as reducing therelative humidity of the external air exposed to the package and/orreducing the storage temperature can further reduce degradation ofcyclic ester product.

POLYMER COMPOSITIONS

A further aspect of the present invention includes the production ofhigh molecular weight polymers from cyclic esters under acceptablepolymerization conditions. It is known that under extreme conditions oflow polymerization temperatures and long polymerization times, highmolecular weights of cyclic ester polymers, such as polylactic acid canbe achieved. Attempts at polymerization at high temperatures overshorter time frames, however, produce lower molecular weight polymers.It is recognized that impurities in cyclic ester starting materialsexacerbate the problem of limited molecular weights at commerciallydesirable polymerization conditions. It is also recognized thatimpurities in cyclic ester starting material decrease the rate ofpolymerization thus increasing the reaction time required to attain agiven conversion. Furthermore, it is also recognized that the degree ofmixing which occurs during polymerization has a dramatic effect on thereaction rate, with higher degrees of mixing giving significantly fasterrates of polymerization. It has been determined that by using, forexample, highly pure cyclic ester compositions of the present invention,high molecular weight polymers can be produced at commerciallyacceptable conditions.

In view of the above, a further aspect of the present invention includesa process for the production of polymers which includes polymerizingcyclic esters at a temperature greater than about 150° C. for less thanabout 30 hours, whereby a degree of polymerization of the resultingpolymeric composition is greater than about 1,700 where XD is themonomer basis for defining degree of polymerization. In furtherembodiments of this process, the temperature can be greater than about160° C. and most preferably is greater than about 170° C. In stillfurther embodiments, the polymerization time is less than about 15hours, preferably less than about 5 hours and most preferably less thanabout 15 minutes. In still further embodiments of the present invention,the degree of polymerization of the resulting polymeric composition isgreater than about of 2,100 and more preferably greater than about2,800.

In preferred embodiments of the polymerization process of the presentinvention, the cyclic esters which are polymerized include the cyclicester composition as described above which can be produced by thepurification processes described herein.

The polymerization process of the present invention can be used toprepare high molecular weight polymers. For example, in the instance ofXD being lactide, polymers having molecular weights in excess of M_(w)=250,000, more preferably molecular weights in excess of M_(w) =300,000,and most preferably molecular weights in excess of M_(w) =400,000 can beproduced. In the instance of XD being tetramethyl glycolide, polymershaving molecular weights in excess of M_(w) =300,000, more preferablymolecular weights in excess of M_(w) =360,000, and most preferablymolecular weights in excess of M_(w) =480,000 can be produced.

CYCLIC ESTER PRODUCTION Processes Directly Converting X₂ A into CyclicEsters

The cyclic ester purification process described above is suitable forpurification of any composition which contains cyclic esters. Althoughthe primary use of the process is for purification of cyclicester-containing compositions from production processes for cyclicesters, the purification process can also be used for purification ofcyclic ester-containing compositions which are from commercial sources.

Suitable cyclic ester production processes are disclosed, for example,in U.S. Pat. No. 5,319,107 and in copending U.S. Ser. No. 08/128,797,the disclosures of which are incorporated herein by reference. Thesedisclosures generally provide for the production of cyclic esters. Onepreferred embodiment includes a process for producing a cyclic ester byproviding a feedstream comprising XA and treating the feedstream to formthe cyclic ester directly from the X₂ A component of XA. In anotherpreferred embodiment, the cyclic ester production process includes (1)providing a feedstream comprising XA in a solvent, and (2) removingwater from the feedstream to form a product stream comprising saidcyclic ester, wherein the concentration of X₅ A and higher oligomers inthe product stream is less than about 20 wt % of the reaction mixtureduring the process. A further preferred process for producing a cyclicester includes (a) providing a feedstream comprising XA diluted in anorganic solvent; and (b) removing water from said feedstream to directlyform said cyclic ester from X₂ A.

Pre-Adsorption Treatment Processes

The various cyclic ester production processes can also include one ormore initial purification steps prior to adsorption purification asdescribed above. A first such initial purification step can includeperforming the cyclic ester production process in such a manner thatafter production the reaction mixture forms two liquid phases inequilibrium to substantially separate XD from impurities. One such phasecontains predominantly the XD and solvent and the second phasepredominantly contains X₁ A and oligomers of X₁ A. The method includesproviding a recovery solvent for the cyclic ester production mixture,and the recovery solvent can be the solvent used for production ofcyclic esters. It should be noted that the second phase can eitherinclude a second phase solvent or consist primarily of X₁ A. Cyclicesters are then recovered from the first phase by adsorption treatmentas described above. The cyclic ester production mixture can furtherinclude soluble esterification catalysts, such as sulfuric acid, whichpreferably partitions into the second phase. In this manner, thecatalyst is readily separated from the cyclic ester.

Any solvent having suitable characteristics in accordance with theabove-described functional parameters for a recovery solvent is suitablefor use in the present process. More particularly, suitable recoverysolvents include xylene, toluene, anisole, benzene, MIBK, and isopropylether, more preferred recovery solvents include xylene and toluene, withxylene being even more preferred.

The step of allowing phase separation of the cyclic ester productionmixture into first and second phases is typically accomplished simply byallowing the mixture to cool with the cessation of any mixing or otheragitation. Alternatively specialized equipment utilizing porous media orelectrical fields can be utilized for the coalescing. These methods canbe done either batch or continuously using standard phase separationequipment known to those skilled in the art.

In addition to separating cyclic esters and solvent from X₁ A andoligomers of X₁ A by allowing phase separation, as an alternativeembodiment, an additional solvent extraction step can be conducted onthe second phase which is rich in X₁ A and oligomers of X₁ A. Thissolvent extraction step is done to recover cyclic esters and solventwhich remain in the second phase. For example, solvent, which istypically the recovery solvent, and the second phase are introduced intoan extraction unit to recover residual cyclic ester and solvent in thesecond phase.

Another embodiment of the present invention, which can be used inconjunction with post-adsorption treatments described above orindependently, includes the introduction of an aqueous solvent to amixture having cyclic esters and impurities and allowing this mixture toseparate into a first phase comprising XD and a second phase comprisingthe aqueous solvent and Impurities, such as XA and other polarimpurities, such as water, in a phase separation. In this embodiment,the amount of aqueous solvent added is less than about 3% (based on theweight of the mixture or composition) above the mutual solubility limitof water in an organic solvent in the mixture. It has been discoveredthat the addition of relatively small amounts of an aqueous solvent,which is a polar material, will (1) help cause a phase split when oneotherwise would not occur, such as where an XD reaction mixture includesa solvent, such as anisole, and (2) provide a more favorabledistribution coefficient between XD and impurities in first and secondphases for solvents, such as xylene, that form separate phases even inthe absence of the aqueous solvent.

Use of the above described water assisted phase separation as apurification technique for XD purification provides a number ofsignificant advantages in integrated XD production and recoveryprocesses. One such advantage is that, in a process for the productionof XD in a non-aqueous solvent and subsequent recovery of XD, the rangeof solvents available for XD production is greatly expanded. By way ofexample, solvents which have unacceptably low distribution coefficientsbetween XD and XA species or do not phase separate in a phase separationstep but which are otherwise desirable either due to a high XDselectivity, conversion and/or productivity, such as anisole, or due toa greater environmental acceptability and/or acceptability for consumerapplications, such as 2-octanone, can now be used.

Another advantage of the water-assisted phase separation process is thatit can simplify subsequent recovery processes. For example, in theinstance of a subsequent adsorption recovery process, the addition of anaqueous solvent in a phase separation step enhances the separation offree acid species from the XD mixture, thereby reducing the demand forremoval of free acid by an adsorbent. Such a reduction reduces eitherthe size of the adsorption units or the needed frequency of adsorbentregeneration cycles and therefore significantly reduces operating andcapital costs. In the absence of the aqueous solvent addition step, theXA levels in the XD containing phase can be relatively high and requirean unacceptably large adsorption unit which must be regeneratedfrequently. The adsorbent regeneration processes noted above can becostly, especially those relying on thermal energy consumption foradsorbent regeneration. The significant reduction in XA and otherimpurity levels in the XD-containing phase achieved by the addition ofthe aqueous solvent permits not only a lower frequency of adsorbentregeneration cycles but also a significant reduction in the amount ofadsorbent used (and therefore the size of the equipment required tocontain the adsorbent).

The aqueous solvent can be any water-based solvent that is nonreactivetowards XD at the desired operating temperatures and concentrations inthe aqueous addition and phase separation steps. The term nonreactivemeans that the aqueous solvent will not cause a significant amount of XDto ring-open. Preferably, less than about 10% of XD is degraded byreaction with the added aqueous solvent, more preferably less than about5%, and most preferably less than about 1%. Suitable aqueous solventsinclude water and solutions containing XA from 0% to 88% by weight. Morepreferred aqueous solvents include water and dilute XA solutions(containing XA from about 0% to about 5% by weight), with water beingeven more preferred. In one embodiment of the water-assisted phaseseparation, the aqueous solvent includes a slipstream portion of thefeedstream to a cyclic ester production unit. In this embodiment, afterphase separation, the second phase comprising aqueous solvent and forexample, XA species, is directed through a hydrolysis step to the cyclicester production unit.

In one embodiment, the aqueous solvent is substantially free of XDimpurities other than water. Such impurities can create problems in thelater processing of the XD and its polymeric derivatives and/or the useand/or disposal of the polymeric derivatives. Impurities include singleor straight-chain hydroxycarboxylic acid or its ester, salt or amide,and other impurities coming from the XA feedstock and/or side productsfrom the XD synthesis reaction step. Alternatively, impurities caninclude many Metal-containing hydroxides, carbonates, and salts (e.g.,hydroxides, carbonates, and salts of alkaline metals and alkaline earthmetals). More preferably, the aqueous solvent includes no more thanabout 1% by weight single- or straight-chain hydroxycarboxylic acid orits ester, salt, or amide. The aqueous solvent further includes no morethan about 1% by weight of other impurities coming from the XA feedstockand/or side products from the XD synthesis reaction step.

It should be noted that the aqueous solvent is itself an impurity thatis detrimental to the stability of XD and subsequent use of XD, such aspolymerization, and residual amounts of the aqueous solvent arepreferably removed as discussed above in a water adsorption step.Therefore, the amount of aqueous solvent contacted with the reactionmixture to form a composition is preferably maintained at the minimumlevel to yield the desired phase separation performance. The amount ofaqueous solvent contacted with the reaction mixture is preferably lessthan about 3% (based on the volume of the total reaction mixture afteraddition of the aqueous solvent) above the mutual solubility limit ofwater in the organic solvent (i.e., less than about 3% above the maximumwater concentration at which water and an organic solvent in thereaction mixture are miscible), more preferably less than about 1.5%,and most preferably less than about 1% by weight based on the weight ofthe composition after addition of the aqueous solvent. It should benoted that the water assisted phase separation can be conducted oneither a continuous or a batch basis. In the instance of a continuousprocess, the above values for the amount of aqueous solvent used refersto the total amount of aqueous solvent added to the total amount of XDcontaining composition for any given time portion during operation ofthe continuous process.

In another aspect, the amount of aqueous solvent added to the reactionmixture is equal to or less than an amount necessary to effecthydrolysis of oligomeric XA species in the reaction mixture in asubsequent hydrolysis operation. Thus, in this aspect of the invention,the amount of aqueous solvent added to the reaction mixture is equal toor less than an amount which provides a stoichiometric amount of waternecessary for the hydrolysis of ester linkages in oligomeric XA species.

The temperatures of the cyclic ester containing composition in the waterassisted phase separation step must be low enough to inhibit significanthydrolysis of the XD by the aqueous solvent and to lower the solubilityof acid impurities in an organic solvent containing the XD to acceptablelevels, yet be high enough to inhibit the precipitation of XD uponaddition of the aqueous solvent. Preferably, the temperature in thewater assisted phase separation step is less than about 80° C. and morepreferably range from about 0° C. to about 60° C. and most preferablyfrom about 25° C. to about 50° C.

The cyclic ester containing composition, which includes both cyclicesters and impurities, can also include an organic solvent. Suchsolvents include, but are not limited to xylene, toluene, benzene,methyl isobutyl ketone, isopropyl ether, anisole and 2-octanone, morepreferably such solvents include xylene and 2-octanone, and mostpreferably such solvents include xylene.

Using the above-noted parameters, the XD in the composition report tothe first phase and impurities report to the second phase. Preferably,the amount of XD reporting to the first phase is at least about 80%,more preferably at least about 90%, and most preferably at least about95% by weight of the XD in the composition. The amount of impuritiesreporting to the second phase is at least about 60%, more preferably atleast about 80%, and most preferably at least about 95% by weight of theimpurities in the composition.

Prior to use of a water-assisted extraction step, as discussed above, afirst extraction step can be performed without the assistance of water.Such a two-step process can acheive higher purities than a one-stepextraction process.

The equipment that can be used for the extraction steps can be any ofthose commonly used for extraction such as mixers-settlers,mixers-coalescers, multi-stage extraction columns (plate, rotating andreciprocating disc), and membrane-based liquid-liquid extraction units.

After separation of the first phase from the second phase by thetechniques described above, the first phase can be treated to purify thecontained XD by techniques described above and the second phase recycledto the feedstream in the cyclic ester production process afterhydrolysis. As noted above, the XD in the first phase can be furtherpurified by adsorption, crystallization, and distillation techniqueswith adsorption being the most preferred technique.

After the water assisted phase separation described above, the firstphase is preferably substantially free of impurities. In particular, thefirst phase contains less than about 100 meq free acid/kg solution, morepreferably less than about 50 meq free acid/kg solution and mostpreferably less than about 10 meq free acid/kg solution. Further, thefirst phase contains less than about 2000 ppm water, more preferablyless than about 500 ppm water and most preferably less than about 250ppm water.

As noted above, the use of a water assisted phase separation providessignificant advantages, however, the process involves the introductionof water into an XD product stream. While water is considered to be animpurity, the introduction of water can be well tolerated if, forexample, a subsequent water adsorption step as described elsewhere isconducted. Even with the use of a water assisted phase separation step,the subsequent use of a water adsorption step can produce cyclic estercompositions with low water contents as described elsewhere herein.

Other Cyclic Ester Production Processes

Other cyclic ester production processes are known and are suitable forproduction of cyclic esters in the present invention. For example,another process for producing a cyclic ester includes vaporizing aportion of a feedstream comprising XA and reacting the vaporized portionof the feedstream in a reaction zone maintained at pressure andtemperature conditions sufficient to maintain the vaporized portion in avaporized state and to form cyclic esters. Cyclic esters can also beproduced such as through the use of depolymerization reactions andcondensation reactions utilizing α-halo salts. For example, acomposition containing polylactic acid can be depolymerized in order toobtain cyclic esters through the use of a "back-biting" mechanism. In anα-halo salt reaction, a cyclic ester is produced by the reaction ofα-halo salt molecules.

INTEGRATED CYCLIC ESTER PRODUCTION AND PURIFICATION

The present invention further includes an integrated process for theproduction of cyclic esters and purification by use of an adsorptiontreatment as broadly described above. A variety of such cyclic esterproduction processes are also described above. In a preferredembodiment, such an integrated process includes (1) providing afeedstream comprising XA in a solvent, (2) removing water from thefeedstream to form a product stream including the cyclic ester, whereinthe concentration of X₅ A and higher oligomers in the product stream isless than about 20 wt % of the reaction mixture during the process; and(3) contacting the product stream with a first adsorbent to remove atleast one impurity from the product stream selected from free acid,water and mixtures thereof to form a purified product stream.

Referring now to FIG. 1, a flow diagram of one embodiment of the presentinvention is shown illustrating an integrated production andpurification process of the present invention for the cyclic esterlactide. In FIG. 1, lactic acid 100 is fed to a hydrolysis unit 102 andcombined with water 104 and an isomer control agent 106 which are alsofed to hydrolysis unit 102. Lactic acid present as L₃ A and higher orderoligomers are hydrolyzed in the hydrolysis unit 102 in order to producea mainly L₁ A and L₂ A feed 108 to LD reactor 110. Used water 112 fromthe hydrolysis unit 102 is purged from the system. Lactic acid in lacticacid feed 108 to the LD reactor 110 is combined with a solvent 114 and,if required, a catalyst 116 both of which are also fed to LD reactor110. Lactide is produced in LD reactor 110 such as by techniquesdescribed previously. Water removed in the lactide production reactioncan be recycled and used as water 104 in hydrolysis unit 102 aspreviously described. The lactide, impurity and solvent mixture 118produced in LD reactor 110 can then be fed to a phase split orextraction process 120 in order to initially remove impurities, (e.g.,XA and XA oligomers) from the lactide and solvent mixture. The XAimpurities removed 122 from the lactide, impurity and solvent mixturecan be recycled back to hydrolysis unit 102. The purified lactide andsolvent mixture 124 can be subjected to a first adsorption 126 to removeat least a portion of XA impurities, including acid impurities, such asthrough the use of an anion exchange resin. The purified lactide andsolvent mixture 128 from the first adsorption can then be fed to asecond adsorption 130 to remove water that may still present in thepurified lactide and solvent mixture 128. The purified lactide andsolvent mixture 132 from the second adsorption can then be concentrated,such as through the use of an evaporation unit 134 in which solvent 136is driven off and can be recycled or combined with additional solvent138 and fed to lactide reactor 110 as solvent stream 114. Theconcentrated, purified lactide and solvent solution 140 obtained fromevaporation unit 134 can be further purified as described above, or, canbe used for the polymerization of lactide into polylactic acid.

The embodiment as described in FIG. 1 also shows the regeneration of theadsorbents in the first adsorption 126 and second adsorption 130.Solvent 142, such as that used as a feed for LD reactor 110 can becontacted with adsorbent in the first adsorption apparatus 126 in orderto desorb impurities on the adsorption material. The resulting solventwhich contains impurities 144 can be utilized in the phasesplit/extraction apparatus 120 for the initial purification of thelactide and solvent mixture 118. Adsorbents used in the secondadsorption 130 can also be regenerated by drying the adsorbents toobtain a water and solvent mixture 146 which can be recycled back tohydrolysis unit 102 as previously described.

Referring now to FIG. 2, a flow diagram of another embodiment of thepresent invention is shown illustrating the addition of the aqueoussolvent in the context of an integrated production and purificationprocess of the present invention for cyclic ester lactide. Lactic acid200 is combined with recycled lactic acid 202 and fed to the lactidereactor 204. Solvent 206 and, if required, catalyst 208 are combinedwith the lactic acid in the lactide reactor 204. Lactide is produced inthe lactide reactor 204 such as by techniques described above. Water 210removed in the lactide production reaction can be recycled. The lactide,impurity, and solvent mixture 212 produced in the lactide reactor 204can then be fed to a phase split or extraction process 214 to initiallyremove impurities, such as XA and water, from the lactide and solventmixture. Aqueous solvent 216 is added to the mixture during the process214. The aqueous solvent 216 can be a portion 217 of the lactic acid200, the water 210, clean water 218 and/or dilute lactic acid comingfrom other process units. The impurities 220 removed from the mixture212 can be sent to a hydrolysis unit 222 and hydrolyzed with water 210to form the recycled lactic acid 202. To maintain desired solvent and XAlevels in the lactide reactor 204, a solvent purge 223 and an XA purge224 can be removed from the hydrolysis unit 222. The purified lactideand solvent mixture 226 can be subjected to a first adsorption 228 toremove at least a portion of the XA impurities, such as through the useof an anion exchange resin. The purified lactide and solvent mixture 230from the first adsorption 228 can then be fed to a second adsorption 232to remove water and aqueous solvent that may still be present in thepurified lactide and solvent mixture 228. The purified lactide andsolvent mixture 234 from the second adsorption can then be concentrated,such as through the use of an evaporation unit 236 in which solvent 238is driven off and can be recycled or combined with additional solvent240 and fed to the lactide reactor 204 as solvent stream 206. Theconcentrated, purified lactide and solvent solution 242 obtained fromevaporation unit 236 can be further purified, as described above, or canbe used for the polymerization of lactide into polylactic acid.

The embodiment in FIG. 2 also shows the regeneration of the adsorbentsin the first adsorption 228 and second adsorption 232. Solvent 244 canbe contacted with the adsorbent in the first adsorption apparatus 228 todesorb the XA on the adsorption material. The resulting solvent 246,which contains desorbed XA, can be combined with the impurities 220 andsent to the hydroyzing unit 222. Adsorbents used in the secondadsorption can be regenerated as discussed above to obtain water 248which can be recycled back to the hydrolysis unit 222 along with thesolvent 246 and impurities 220.

The following examples and test results are provided for the purposes ofillustration and are not intended to limit the scope of the invention.

EXAMPLES Example 1

The following example illustrates the effectiveness of one embodiment ofthe purification process of the present invention for obtaining purifiedcyclic esters of hydroxycarboxylic acids.

Cyclic Ester Production

A 5 g sample of racemic 2-hydroxyoctanoic acid, 0.22 g of Dowex-50cation exchange resin in the acidic form (as a reaction catalyst), and195 ml of toluene were added to a 3-neck flask fitted with a heatingmantle, pot thermometer, magnetic stirrer, Dean-Stark tube, refluxcondenser, and a rubber septum. The mixture was heated to reflux at atemperature of about 116° C. and held there for about 48 hours.

Aliquots of the reaction mixture were derivatized with diazomethane andanalyzed by gas chromatography-mass spectrometry (GC-MS). Separate peakswere identified for: 2-hydroxyoctanoic acid, the linear ester formedfrom 2 molecules of 2-hydroxyoctanoic acid; the meso isomer of thecyclic diester of 2-hydroxyoctanoic acid; the D and L isomers of thecyclic ester of 2-hydroxyoctanoic acid; and a longer retention time peakfor an unknown compound which was thought to be one or more higherlinear esterification products.

Cyclic Ester Purification

A 300 ml sample of AMBERLYST® A-21 ion exchange resin was used as anadsorbent. The resin was first washed with several acetone rinses. Theresin was placed in a 1 inch diameter column and then washed with a 1:1mixture of toluene:acetone solution to displace any initial water heldby the resin. The cyclic ester reaction mixture was diluted with acetoneto a 1:1 toluene:acetone mixture and passed over the previously preparedAMBERLYST® A-21 ion exchange resin bed. The effluent was collected andevaporated to dryness. The solid product was analyzed by gaschromatography. Two peaks were found: the meso isomer of the cyclicdiester of 2-hydroxyoctanoic acid, and the D and L isomers of the cyclicester of 2-hydroxyoctanoic acid. Thus, purification of the cyclic esterwas achieved in a non-stereospecific process, thereby allowing forrecovery of a mixed isomeric population of cyclic esters whichcorresponds to the isomeric makeup of the feed to the adsorptionprocess. The acid and linear esterification products of thehydroxycarboxylic acids were not found. A total of 0.6629 g of solidproduct was obtained giving an isolated yield of 34% of the theoreticalyield calculated on a molar basis.

Example 2

The following example illustrates the production and purification ofcyclic esters according to one embodiment of the present invention andsubsequent production of polymers and also provides two comparativepurification processes and attempts at polymerization.

Cyclic Ester Production

Crude tetramethyl glycolide was prepared by direct liquid phasesynthesis from 2-hydroxy-2-methylpropionic acid using m-xylene as thesolvent and p-toluene sulfonic acid as the catalyst. The mixture wasrefluxed using a Dean-Stark tube to remove water.

2.1 Cyclic Ester Purification and Polymerization

The crude tetramethyl glycolide was washed with sodium carbonate,neutralized, dried, and then dissolved in acetone. The crudeteteramethyl glycolide/acetone solution was passed through a columncontaining AMBERLYST® A-21 ion exchange resin as an adsorbent to removeacid impurities. The column effluent was evaporated to dryness. Theresulting solid was recrystallized using petroleum ether and dried. Asample of the resulting tetramethyl glycolide was analyzed bydifferential scanning calorimetry and was found to have a sharp meltingpoint with onset at 80.9° C.

The purified tetramethyl glycolide was polymerized using lithiumtert-butoxide catalyst for about 8 hours at about 130° C. The molecularweight of the resulting polymer was determined by gel permeationchromatography to have a M_(w) =520,000 and a M_(w) =354,000.

2.2 First Comparative Cyclic Ester Purification and Polymerization

The crude tetramethyl glycolide was washed with sodium carbonate,neutralized, and dried. The crude tetramethyl glycolide was dissolvedand recrystallized using isopropanol as the solvent. A sample of therecrystallized material was analyzed by differential scanningcalorimetry and was found to have a broad melting point with onset at79.0° C.

A first attempt at polymerization of the purified material usingstannous octoate as a catalyst failed. No detectable polymerizationoccurred and unreacted tetramethyl glycolide was recovered after a longreaction period.

A second attempt at polymerization of the purified material waspartially successful. The polymerization was carried out at 130° C.using lithium tert-butoxide for the catalyst. The molecular weight ofthe resulting polymer was determined by gel permeation chromatography tohave Mw=17,200 and Mn=14,525. Only 42% of the starting monomer wasconverted to polymer.

The low molecular weights and poor conversions in this comparativeexample indicate that the solvent recrystallized tetramethyl glycolidewas not of sufficient purity to produce high molecular weight polymer.In contrast, the adsorption based purification described in Example 2.1was able to produce high purity tetramethyl glycolide suitable forproducing high molecular weight polymer.

2.3 Second Comparative Cyclic Ester Purification and Polymerization

The crude tetramethyl glycolide was washed with sodium carbonate,neutralized, dried, then dissolved in acetone. The crude tetramethylglycolide/acetone solution was passed through a resin bed containingAMBERLYST® A-21 ion exchange resin to remove any residual acid species.The column effluent was evaporated to dryness. The resulting solid wasrecrystallized from isopropanol and dried. A sample of the resultingtetramethyl glycolide was analyzed by differential scanning calorimetryand was found to have a sharp melting point with onset at 81.0° C.

An attempt at polymerizing the purified tetramethyl glycolide at 130° C.using lithium tert-butoxide catalyst failed. After eight hours, therewas little evidence of polymerization. Residual traces of isopropanol inthe purified tetramethyl glycolide are believed to have caused thefailure of the polymerization. While drying of the purified tetramethylglycolide removed the solvent adhering to the exterior of the crystals,it most likely did not remove the solvent trapped inside the occlusionsof the crystals. It was, therefore, believed that there were tracelevels of alcohol in the purified material.

The sharp melting point of this example indicates that the purity of thetetramethyl glycolide was comparable to the purity of the tetramethylglycolide in Example 2.1 This example shows that even when an adsorptionbased process is followed by a positive purification step like solventcrystallization, that use of an alcohol solvent for crystallizationintroduced alcohol impurities into the cyclic ester composition whichcaused failure of polymerization.

Example 3

The following example illustrates the recovery of a mixed isomer lactideproduct to one embodiment of the purification and recovery process ofthe present invention.

Cyclic Ester Production

A 1025 g sample of mixed xylene, 50 μl of sulfuric acid, and someboiling chips were placed into a 2000 ml three neck flask fitted with aDean-Stark tube, condenser, and heating mantle. The contents were heatedto reflux and 50 ml of lactic acid solution was added. The lactic acidsolution was made from 9 parts of 88% L-lactic acid and 1 part of 88%racemic lactic acid for an overall L:D ratio of 95:5 in the startingmaterial. The reaction mixture was boiled at atmospheric pressure forabout 4.5 hours during which time a total of 15 ml of water werecollected in the Dean-Stark tube. The mixture was cooled to roomtemperature and allowed to phase split.

Cyclic Ester Purification

A 977 g sample of the upper phase material which contained lactide wasadded to a jar which contained 214 g of previously azeotropicallydistilled REILLEX® 425 ion exchange resin as an acid adsorbent, 144 g of3A molecular sieves as a water adsorbent, and 50 g of activated carbonas an adsorbent for color bodies. This mixture was stirred for about 4hours and a sample of the solution was taken and filtered through a 0.45μm filter prior to analytical analysis.

High performance liquid chromatography (HPLC) analysis of the solutionshowed no detectable free acids. The lower detection limit of the HPLCmethod in this application was 2 meq free acid/kg of solution. The HPLCresults also showed that the L:D:M isomer ratio of the material wasunchanged by the adsorption treatments. Karl-Fisher titration showedthat the treated solution contained 16.6 ppm of water.

Recovery of Mixed Isomer Lactide Product

The treated solution was then rotavaped to dryness to simulate theoperations of a spray dryer. The cake residue was a white snow-likepowder. Analysis of the cake showed that the L:D:M isomer ratio was96.70:1.04:2.26, which indicates that a portion of the M-lactide waslost during the evaporation. This is not surprising given the highervapor pressure of the M-lactide compared to the other isomers. A simplestripping section could be used to retain the M-lactide in industrialequipment. High performance liquid chromatography analysis of the powdershowed no detectable free acids. The lower detection limit of the HPLCmethod in this application was 9 meq free acid/kg of lactide.

Example 4

The following example illustrates one embodiment of the regenerationprocesses for the adsorbents of the present invention.

An adsorption apparatus consisting of four 1" diameter insulatedstainless steel tubes, each about 5 feet long, mounted vertically on acart and connected in series. Sample ports were installed in the feedline, between each column, and in the effluent line. REILLEX® 425 resinwas prepared to remove any water by azeotropic distillation with xylene.The four columns were filled with 311.6 g, 356.4 g, 370.1 g, and 352.3 gof wet resin, giving a total bed length of about 16 feet. Fresh xylenewas pumped into the apparatus and air was bled from the system.

First Adsorption

A room temperature feed solution containing xylene, lactic acid, linearlactic acid esters, and lactide was pumped through the adsorptionapparatus in an upflow direction at about 45 g/min, giving an upflowsuperficial velocity of 0.39 ft/min. Samples were taken every half hourat the five sample ports and submitted for high performance liquidchromatography (HPLC) analysis. After the initial xylene was displacedfrom the system, samples of the fourth column effluent showed nodetectable levels of free acids until breakthrough of the column.Breakthrough of the fourth column occurred about 270 minutes after thefeed solution was started. The lower detection limit of the HPLC methodin this application was 2 meq free acid/kg of solution. An 807 g retainsample of the purified stream was taken from the fourth column prior tobreakthrough. A 178 g sample of 3A molecular sieves was added to theretain sample. The retain sample was then stirred and stored for lateruse.

At about 270 minutes and beyond, the acid components broke through thefourth column in reverse molecular weight order. This observationindicates that the resin capacity decreases as the chain length of thelinear lactic acid esters increases. The feed solution was stopped atabout 480 minutes after it was started, at which time nearly completesaturation of the resin was observed.

Adsorbent Regeneration

The resin from the first column was removed and placed into a three neckflask fitted with a heating mantle and a condenser. Enough xylene wasadded to cover the resin, and the mixture was heated until boiling atabout 133° C. After about a few minutes of boiling, the resin wasfiltered while hot. The resin was boiled in the same manner a secondtime in xylene, filtered, then placed back in the column. This thermalregeneration procedure was repeated for the second, third, and fourthcolumns. Fresh xylene was then pumped into the apparatus and air wasbled from the system.

Second Adsorption

A second room temperature adsorption cycle was performed using the samexylene/lactic acid/lactide solution used in the first adsorption cycle.The flowrate for the second adsorption cycle was about 47 g/min, givingan upward superficial velocity of 0.41 ft/min. Samples were taken everyhalf hour and submitted for HPLC analysis. After the initial xylene wasdisplaced from the system, samples of the fourth column effluent showedno detectable levels of free acids until breakthrough of the column.Breakthrough of the fourth column occurred about 240 minutes after thestart of the feed solution. As in the first adsorption cycle, the lowerdetection limit of the HPLC analysis in this application was 2 meq freeacid/kg of solution. A 2580 g retain sample of the purified stream wastaken from the fourth column prior to breakthrough. A 570 g sample of 3Amolecular sieves were added to the retain sample. The retain sample wasthen stirred and stored for later use.

Since (1) the first adsorption cycle was carried out until near completesaturation of the resin, and (2) the length of the second adsorptioncycle prior to breakthrough was about as long as the first adsorptioncycle, the thermal regeneration of the resin was considered to have beensubstantially complete.

The two retain samples from first adsorption and second adsorptioncycles were combined and a portion of the xylene was removed byrotavaping the material at 70° C. and partial vacuum. After rotavaping,the residue was cooled to room temperature during which time whiteneedle-like crystals of L-lactide formed in the residue flask. Thecrystals were filtered, washed with fresh xylene which had been driedusing 3A molecular sieves, then dried at 35° C. in a vacuum oven. Thefree acid level of the crystalline product was below the detection limitof a potassium methoxide titration method. The detection limit of thepotassium methoxide titration method in this application was 0.02 meqfree acid/kg of lactide.

Example 5

The following comparative example illustrates the high molecular weightof polymers which can be produced using the purified cyclic estersobtained according to one embodiment of the production and purificationprocesses of the present invention relative to the molecular weight ofpolymers which can be obtained using commercially available cyclicesters.

Adsorbent Preparation

The first two tubes of the adsorption apparatus described in Example 4were filled with REILLEX® 425. The REILLEX® 425 had been previouslyprepared by azeotropic distillation with xylene. The third and fourthtubes were filled with 3A molecular sieve beads. Xylene was pumped intothe system, the molecular sieves allowed to degas, and the air purgedfrom the columns.

Cyclic Ester Purification

A sample of 85 g of L-lactide were dissolved in 4 liters of xylene. TheL-lactide was a commercial grade believed to be made from adepolymerization-based reaction. Prior to dissolution, the lactide wasexposed to air for an extended period and allowed to pick-up water andform free acid. The starting lactide contained 91 meq free acid/kglactide as determined by titration with potassium methoxide. Karl Fishertitration of the starting lactide, the starting xylene andlactide-xylene solution gave 260, 78 ppm and 84 ppm water, respectively.

The crude lactide-xylene solution was pumped through the adsorptionapparatus at a rate of 46 g/min. After the initial xylene was displacedfrom the system, a 4 liter retain sample was taken from the fourthcolumn effluent. High performance liquid chromatography (HPLC) analysisof the retain sample showed no detectable free acids. The lowerdetection limit of the HPLC method in this application was 2 meq freeacid/kg solution. Karl Fisher titration of the retain sample showed 7.7ppm of water.

Recovery of Cyclic Esters

A portion of the xylene from the retain sample was removed by rotavapingthe material at 65° C. under partial vacuum. After rotavaping, theresidue was cooled to room temperature during which time whiteneedle-like crystals of L-lactide formed in the residue flask. Thecrystals were filtered, washed with fresh xylene which had been driedusing 3A molecular sieves, then dried at 35° C. in a vacuum oven. Thefree acid level of the crystalline product was determined to be 0.04 meqfree acid/kg lactide by potassium methoxide titration. Karl Fishertitration of the crystalline product showed 49 ppm of water.

Polymerization of Cyclic Esters

Six glass tubes were each filled with 10 grams of either the adsorptionpurified lactide, a commercial lactide, or the starting lactidematerial. The commercial lactide contained 1.1 meq free acid/kg lactideand 59 ppm water as determined by potassium methoxide titration and KarlFisher titration. A 24 μl sample of a 10 wt % solution of stannousoctoate in xylene solution was added to each tube, which corresponds toa catalyst level of 80 ppm molar or a 12500:1 monomer to catalyst ratio.The tubes were heated and the contents allowed to polymerize. Themolecular weights shown below were determined by gel permeationchromatography analysis:

    ______________________________________    Tube  Material      T (°C.)                                t (hr)                                      Mw    Mn    ______________________________________    1     Purified lactide                        167     4     399,800                                            255,325    2     Commercial lactide                        167     4     268,700                                            130,135    3     Starting lactide                        167     4     15,100                                            11,668    4     Purified lactide                        145     44.5  441,100                                            179,600    5     Commercial lactide                        145     44.5  196,400                                            94,200    6     Starting lactide                        145     44.5  30,200                                            29,900    ______________________________________

Since the molecular weights of the polymers derived from the purifiedlactide are about 2000% higher than those for the polymer derived fromthe starting lactide, the purification process was successful. Thepolymers from the purified lactide were 50-125% higher in molecularweight when compared to the polymers from the commercial lactide. Sincethe polymerization conditions were identical, the difference inmolecular weights are due to differences in the monomer (lactide)purity.

Example 6

The following example illustrates another embodiment of the purificationprocesses of the present invention.

Adsorbent Preparation

One hundred ml of dry AMBERLYST® A-21 resin was placed in a 1 inchdiameter clean glass column. It was rinsed twice with 100 ml portions ofacetone and once with 100 ml of 50 percent water/50 percent acetone. Thefirst acetone rinse had a pH of 9 and was yellow, indicating impuritiesin the resin that were removed by this treatment. Magnesium sulfate (15g) and 40 ml of 3A molecular sieves were added to the top of the column.

Adsorption of Impurities

A 10 g sample of L-lactide, 5 g of anhydrous lactic acid, 2 g polylacticacid and about 0.5 g distilled water were mixed with about 20 ml acetoneand added to the column containing the adsorbents as prepared above. Thelactide preparation was eluted from the column. The eluted fraction fromthe column was placed on a rotary evaporator while gently applying heatto remove the solvent. Residual acetone was further evaporated byplacing the fraction in an oven over phosphorus pentoxide. A very white,light flaky material remained as a residue after the remaining acetonewas driven off. The yield of lactide obtained was approximately 93.5%.

Example 7

The following example illustrates the improved shelf life and stabilityof a packaged cyclic ester product of the present invention.

One 5 kg sample of lactide is added to a 4 mil thick low densitypolyethylene (LDPE) bag having a water vapor transmission rate of about0.39 g/(100 in² ·24 h) and another 5 kg sample of lactide is placed in aLDPE-foil laminate bag having a water vapor transmission rate of about0.0006 g/(100 in² ·24 h). Both bags have 418 square inches of surfacearea. The bags of lactide are then stored at 100° F. and at 90% relativehumidity for six months. The free acid level in the lactide in the 4 milthick LDPE bag is measured and is found to have risen by 3332 meq freeacid/kg lactide by the water transmitted through the LDPE bag. Incontrast, the free acid level in the lactide in the LDPE-foil laminatebag is measured and is found to have risen by only about 5.4 meq freeacid/kg lactide by transmitted water vapor.

Example 8

The following example illustrates the effectiveness of the aqueoussolvent in removing lactic acid and its oligomers and water into aseparate phase from the solvent and lactide.

A stream from the lactide reactor containing 1.139% by weight lactideand 0.728% by weight lactic acid and lactic acid oligomers with theremainder being xylene was contacted with 0.6% by weight water. Themixture was split into two separate phases, with one phase containinglactide and xylene and the other phase containing lactic acid, lacticacid oligomers, and water. The former phase was sampled and found tocontain 1.127% by weight lactide and 0.05% by weight lactic acid andlactic acid oligomers and the latter phase 0.626% by weight lactide and55.1% by weight lactic acid and lactic acid oligomers. Based on theseconcentrations and the relative volumes of the two phases, the formerphase contained 99% of the lactide and 7% of the lactic acid and lacticacid oligomers.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. It is to beexpressly understood, however, that such modifications and adaptationsare within the scope of the present invention, as set forth in theappended claims.

What is claimed is:
 1. A process for the purification of cyclic estersin a composition comprising cyclic esters, an organic solvent andimpurities, comprising:introducing an aqueous solvent into saidcomposition to form a mixture; allowing said mixture to separate into afirst phase comprising said cyclic esters and organic solvent and asecond phase comprising said aqueous solvent and impurities, wherein theamount of aqueous solvent introduced into said composition is less thanabout 3% based on the weight of the composition above the mutualsolubility limit of water in the organic solvent; and further purifyingsaid cyclic ester in said first phase.
 2. The process of claim 1,wherein said further purification is selected from the group consistingof adsorption, crystallization, distillation, and combinations thereof.3. The process of claim 1, wherein said impurities removed by saidsecond adsorbent comprise water.
 4. A process for purification of cyclicesters in a composition comprising cyclic esters, an organic solvent andimpurities, comprising:(a) introducing an aqueous solvent into saidcomposition and allowing said composition to separate into a first phasecomprising said cyclic esters and organic solvent and a second phasecomprising said aqueous solvent and impurities, wherein the amount ofaqueous solvent introduced into said composition is less than about 3%by weight of the composition above the mutual solubility limit of waterin the organic solvent; and (b) contacting said first phase with a firstadsorbent to remove impurities.
 5. The process of claim 4, wherein saidimpurities comprise free acid and said first adsorbent comprises ananion exchange resin.
 6. The process of claim 4, further comprising thestep of:(c) contacting said first phase with a second adsorbent toremove impurities after said step of contacting with said firstadsorbent.
 7. The process of claim 6, wherein said impurities comprisewater and said second adsorbent is selected from the group consisting ofmolecular sieves and alumina and silica gels.
 8. The process of claim 6,wherein said composition before the introducing step comprises no morethan about 1% by weight aqueous solvent.
 9. The process of claim 6,wherein the amount of aqueous solvent introduced into said compositionis less than about 3.0% by weight based on the weight of the compositionabove the mutual solubility limit of water in the organic solvent. 10.The process of claim 6, wherein the amount of said cyclic esters in saidfirst phase is at least about 80% of the cyclic esters in saidcomposition.
 11. The process of claim 6, wherein at least about 60% byweight of said impurities in said composition separate into said secondphase.
 12. A process for the purification of cyclic esters in acomposition comprising cyclic esters and impurities, wherein saidimpurities comprise XA composed of a compound selected from the groupconsisting of a hydroxycarboxylic acid, its ester, salt and amide (X₁A); a linear dimer molecule of X₁ A (X₂ A); a linear trimer molecule ofX₁ A (X₃ A); a linear tetramer molecule of X₁ A (X₄ A); a linearpentamer molecule of X₁ A (X₅ A); and mixtures thereof,comprising:introducing an aqueous solvent into said composition to forma mixture allowing said mixture to separate into a first phasecomprising said cyclic esters and a second phase comprising said aqueoussolvent and impurities, wherein the amount of aqueous solvent introducedinto said composition is less than about 3% by weight of the compositionabove the mutual solubility limit of water in the organic solvent; andfurther purifying said cyclic ester in said first phase.
 13. The processof claim 12, wherein said further purification is selected from thegroup consisting of adsorption, crystallization, distillation, andcombinations thereof.
 14. The process of claim 12, wherein said furtherpurification comprises contacting said first phase with a firstadsorbent to remove impurities.
 15. The process of claim 14, whereinsaid further purification comprises contacting said first phase with asecond adsorbent to remove impurities after said step of contacting withsaid first adsorbent.
 16. The process of claim 15, wherein saidimpurities removed by said second adsorbent comprise water.